Hi,
BookLink needs to be a web address. It can be either a web address that matches the actual location of the file on the server. It can be also a web address that will run some code to grab and send back the file content (if it is stored at a location which is not directly reachable by the internet or in a db or whatever).
Here it seems BookLink should look like for example like /Books/myfile.pdf. something like c;\websites\mysite\books\myfile.pdf won't work as a physical location on the web server c: drive doesn't make sense client side.